Modern inboard boat technology includes several types of drive units that are suitable for providing propulsion to large marine vessels, namely, inboard-fixed strut drive and pod drive. Both drive units are similar in that an engine is rigidly mounted inside the vessel to a hull structure (a.k.a. stringer system) along the hull, and a drive or shaft system is also rigidly mounted separately to the hull so that power can be applied through the shaft system and the resulting propulsive forces can be channeled through the hull structure to propel the vessel.
The inboard-fixed strut drive system includes an engine powering a transmission that is coupled with a propeller shaft having a propeller at an end. In the fixed strut system, the propeller shaft is in a “fixed” position about the vessel bottom, preventing any horizontal or vertical changes relative to the bottom of the hull. Therefore, the vessel operates at all times with the propeller shaft rotating only about its longitudinal axis for propulsion. This system prevents the inboard-fixed strut drive from providing any vessel steering capability and therefore a rudder system is required to steer the vessel.
The pod drive, also known as Azi-pods, were traditionally self contained power units (usually electric), and in contrast to the inboard-fixed strut drive, each pod could “Azimuth” or change steering angles in order to direct thrust (propulsion) or vector the thrust at any desired steering angle. With the Azi-pod, a structure holds the drive to the vessel in a manner that constrains the drive to steer about a fixed steering axis. Although the drive may be allowed to steer through 360 degrees along its steering axis, the steering axis is fixed to the hull and cannot be altered. Therefore, the Azi-pod drive has a steering axis and thrust vectors that are fixed substantially 90 degrees or orthogonally located relative to the underlying vessel bottom surface.
Eventually, a variant of the pod drive was introduced that utilized an engine and transmission mounted outside the pod. As the engine mounting and the pod mounting are separate, the pod mounting allows all the propulsive force to be transmitted directly into the stringer system. In this configuration, a steering axis is created and constrained by a “well” that is constructed inside the stringer system extending through the vessel bottom. The pod drive is then contained and sealed with a double O-ring system that is forcibly held inside the well with a clamp ring. All propulsive and steering forces are transmitted through this O-ring-well system. The steering axis is substantially perpendicular to the vessel bottom or the dihedral angles of the vessel bottom; therefore the pod drive is constrained to steer on the dihedral angle of the vessel bottom. When this drive is mounted to a point where the vessel bottom is not horizontal, this configuration introduces a proportional vertical component of thrust as the pod drive is steered about the steering axis. Additionally, a single piece grommet that constrains and seals the pod about the vessel bottom can be used instead of the O-ring system.
Current inboard boats are controlled on the three axes of freedom, yaw, pitch, and roll, by two systems acting independently, the steering system and the trim system. Both the pod and inboard-fixed shaft drive units can utilize trim tabs to control vessel pitch (trim). The trim tabs can be fixed directly onto the pod or mounted to the stern of the vessel. In addition, or in place of the trim tab, an interceptor can be utilized to provide pitch control. Trim tabs or interceptor blades are typically fastened to the stern of the vessel at the intersection of the bottom surface of the vessel and the stern. The trim tab and interceptor devices are deployed downward at the surface of the water immediately leaving the bottom of the vessel. This downward motion causes a positive upstream pressure to react on the device and the vessel bottom immediately adjacent to the device. This positive pressure causes a lift reaction that raises the stern of the vessel while underway. This stern lift is the control of pitch for inboard planing hulls. Exerting the device against the surface of the water creates a parasitic drag force that reduces thrust efficiency and vessel speed.
As with the trim tab, the use of two pod drives can provide another method of pitch control, although it is also problematic. More particularly, pitch control could be provided when a pod drive is mounted on the port side of a hull that is not horizontal, for example 20 degrees off the horizontal, and another pod drive is mounted on the starboard side which is also 20 degrees off the horizontal, such that their steering axes are angled towards each other and are not vertical. In this case, if both drives are “toed in” such that the vertical thrust components would be added to create a slight net downward force on the stern. If the drives were “toed out,” a net upward force would be created tending to lift the stern. Therefore, pitch control could be gained by a dynamic toe adjustment inward or outward. (Toe adjustment is described as an adjustment from dead forward on both drives of equal magnitude causing the leading point of the gear cases (about the front of the pod) to be closer (toe in) or farther (toe out) apart). Although pitch control can be obtained in this manner, a practical problem with this method of trim is that in order to trim the vessel, forward thrust must be attenuated. Additionally, toeing the gear cases causes increased drag. Moving the thrust vector away from dead forward, and increasing the drag of the drive system, as described to attain trim has an attenuating effect on total forward thrust. Therefore, this method may be just as inefficient or possibly even worse than using trim tab or interceptor methodology.
Adjustment of the pitch (trimming) of a vessel has a substantial effect on the efficiency of the planing boat hull. Recreation marine craft (smaller vessels) for the most part use a planing hull, as these best fulfill the market desire to achieve speeds in excess of 30-40-50 mph. For these speeds, vessel hulls from 12 feet in length to 50 feet in length are designed to be planing hulls. This method requires the least power for the most speed as the vessel is “skimming” over the water as compared to “plowing” through the water as in the case of very large vessels. The dynamic of a planing hull is that it has two states, off-plane and on-plane. The state of the hull dynamic is directly proportional to the speed of the hull in the forward direction. In the off-plane speed range, the vessel is viewed as a displacement hull (like a very large vessel). In this case, the longitudinal keel line is parallel to the keel line when the boat is at rest. As speed is increased, the bow of the vessel rises due to increasing water pressure from speeding forward, causing the wetted surfaces to move aft. As this tendency continues, the wetted surface will move far enough aft until the center of gravity of the vessel causes the vessel to “fall forward” into the planing position.
The stable planing attitude for most hulls will be 4 to 5 degrees bow up compared to the horizontal. In the inboard-fixed drive, the inboard thrust vector is in line with the propeller shaft, which is usually upward at 10 to 13 degrees. With the pod drives, the thrust vector is substantially horizontal (0 degrees). Therefore, when the hulls are on plane at 4 to 5 degrees above the horizontal, this must be added to the fixed thrust angle to understand the dynamic planing state. Thus, the planing inboard-fixed drive thrust angle would range from 14 degrees to 18 degrees above horizontal where the pod drives would be 4 to 5 degrees above horizontal. As the thrust in the horizontal plane causes forward motion, these angles above the horizontal cause the attenuation of forward thrust by the cosine of the angle.